18 February 2009

Do male crickets shorten the lives of female crickets?

One of the deep insights from evolutionary thinking in the fields of ethology, animal behaviour, behavioural ecology, and sociobiology is that males and females often have distinctly different best interests when in comes to reproduction. Now, it’s very easy to made wild extrapolations about this, particularly regarding humans, but there are many examples of the general principle. Of course, when you're looking at any individual case, the devil is always in the details.

This paper, by Green and Tregenza, looks at whether male crickets (Gryllus bimiculatus; sometimes known as “bimac” in cricket labs) manipulate female crickets by transferring special seminal proteins along with sperm when the copulate. The underlying idea is that it is in the best interest of the male for the female not to mate with other males, because this would cut into the first male’s reproductive success. Thus, a male that can change a female’s behaviour or attractiveness or what have you so that she is less likely to mate would have an evolutionary advantage.

Green and Tregenza test this idea by taking spermatophores from several male crickets and making a liquid that they injected into the females. As a control, they injected a different group of females using a physiological saline (water and a few salts that are similar to that inside the cricket).

One of the potential shortcomings of this study is that these were not delivered into the reproductive tract, which obviously would be the case with actual mating, but into the abdomen directly. On the one hand, this is clearly not a natural situation. On the other hand, it does help test whether any effects are due to the physical act of mating rather than actual chemical signals.

The authors find that there are two main effects. Females injected with the mix of seminal proteins walk forward less, and they have shorter lives. The females in both conditions lay about the same number of eggs. Green and Tregenza also looked at other factors, like responses to male song, but found no significant differences. From this, they conclude that the seminal proteins are having manipulative effects on the females.

The discussion does talk about the suggestion that the injection of chemicals is causing a general immune response, but they downplay this, saying that they would have expected to see a greater decline in female’s orientation if there was a general immune response. But it seems a fairly frail argument. This experiment could have made its case much more strongly by adding one more control group.

Imagine that you have the hypothesis that the secret ingredient in a particular cola flavoured soft drink – let’s call that secret ingredient, say, 7X – causes burping. So you give one group of people glasses of water to drink, and one group the one brand of soft drink. The latter burps more. You thus conclude that 7X causes burping, right? Not so fast!

A soft drink is a complex concoction of carbonation, sugars, and flavourings. Water is not. What you would really want as a control is a soft drink with everything but 7X, either instead of, or in addition to, water.

In this paper, a complex protein mixture is being compared to a physiological saline. The implication is that the effects are due to particular proteins generated by the males that function to manipulate the females. But it may be that this is simply an effect of injecting a complex mixture of proteins, and there is no protein specific to males that is having this effect. A second control containing proteins unrelated to crickets, like bovine serum albumin (BSA), would help to sort this out this possibility. Or, use seminal proteins from a distantly related cricket species. If this was really a case of sexual manipulation, you would predict that only the proteins from the same species would have a significant effect.